This invention relates to lasers, and more particularly relates to techniques for providing compensation of group velocity dispersion in lasers.
Group velocity dispersion is an inherent and well understood characteristic of various laser elements including, for example, the laser gain medium, and this dispersion tends to cause broadening of a short laser pulse. In a modelocked laser, the various frequencies composing short laser pulses generated in the laser cavity tend to disperse when traveling through the cavity due to the positive dispersion of the laser gain medium, which in this case might be, e.g., a titanium:sapphire gain medium.
To facilitate such short pulsed operation in modelocked lasers, and indeed to reduce or eliminate group velocity dispersion generally, negative dispersion is intentionally introduced into the laser cavity, typically using an element or set of elements designed specifically to compensate for the positive dispersion of other intracavity elements. Perhaps the most common compensation technique uses prism sequences; one or more pairs of appropriately arranged prisms can result in a geometric variation of laser beam path length with wavelength that is equivalent to negative dispersion. The particular orientation of the prisms within each pair ensures that the dispersion is of the appropriate polarity. Use of compensating prism pairs has been analyzed first in femtosecond dye lasers, by Fork et al., in "Negative dispersion using pairs of prisms," Optics Letters, 9, 150, 1984, and has subsequently found widespread use in, e.g., femtosecond solid-state lasers.
Beyond compensation of group velocity dispersion, it is often desirable to prespecify a net negative dispersion in a laser to achieve, for example, soliron-like pulse shaping in a passively modelocked laser. Fork et al., in "Negative dispersion using pairs of prisms," Optics Letters, 9, 150, 1984, have shown theoretically that by varying the spacing between a pair of prisms and the optical path length of a laser beam passing through the prisms, the amount of net group velocity dispersion within a laser cavity can be varied. Negus et al., in U.S. Pat. No. 5,097,471, have demonstrated this using of a pair of prisms to both compensate for positive group velocity dispersion of a gain medium and to further produce a net negative group velocity dispersion in the laser cavity; Negus and several others have demonstrated this in Kerr Lens Modelocked Ti:Sapphire lasers. Kafka et al., in U.S. Pat. No. 5, 185,750, have also demonstrated the use of a compensating prism pair, using two Brewster prisms placed in a laser cavity at Brewster's angle and arranged so that the second prism recollimates the divergent spectral components coming from the first prism.
Gordon et al., in "Optical resonator with negative dispersion," Optics Letters, 9, 153, 1984, have proposed an alternative approach for dispersion compensation using a laser geometry involving a theoretical double-mirror, ring resonator design that incorporates one intracavity prism for producing a prespecified resonator dispersion of either polarity. Gordon shows that based on proper positioning of the prism, the ring resonator geometry can allow for co-existence of more than one monochromatic laser mode, each with a displaced propagation axis, and so can provide a large degree of negative dispersion, if the cavity lengths are appropriately chosen.
While such a prism scheme and the prism pair schemes described above for laser cavities are recognized as effective means of dispersion compensation, these schemes pose serious constraints on laser geometry and capabilities. The primary limitation is that of complexity and the constraints in design imposed by requiring two or more extra intracavity elements, namely, the compensating prisms, separated by a substantial distance, in addition to the gain, focusing, and reflective end elements inherently required of a laser design. These additional elements increase intracavity loss, alignment complexity, and the cost of manufacture. Commonly used prism materials such as LAKL21 glass, fused silica, or SF10 glass require, by the nature of their dispersive properties, a significant inter-prism spacing to achieve the requisite negative dispersion. Indeed, the double-mirror ring resonator of Gordon described above would require a cavity length in excess of 2 meters to compensate for the material dispersion of just 4 mm of quartz, which is less dispersive than the commonly used Ti:Sapphire gain medium. As a result of such prism separation requirements, femtosecond Kerr Lens Modelocking lasers have historically always used a so-called folded X or Z cavity configuration, with a gain medium located in a converging fold and a prism sequence located in one arm of the cavity. This geometry and prism separation requirements have constrained femtosecond Kerr Lens Modelocking lasers to date to operation in the 100 MHz repetition rate range.
As an alternative group velocity dispersion compensation scheme, Kafka et al., in U.S. Pat. No. 5,185,750, have demonstrated the use of a Gires-Tournois interferometer positioned intracavity. While such a scheme is shown to be effective, a Gires-Tournois interferometer is limited in its ability to operate over wide bandwidths, and consequently cannot be readily used in, e.g., extremely short pulse lasers. Furthermore, interferometers are inherently delicate structures and thereby increase the complexity and cost of lasers they are used in.
Recently, Stingl et al., in "Generation of 11 fs pulses from a Ti:Sapphire laser without using prisms," Optics Letters 19, 204, 1994, have demonstrated the use of mirrors, rather than prisms, as compensation elements; interference of light reflected from multi-layer dielectric stacks at the mirrors' surfaces provides the requisite negative dispersion. Like the other dispersion compensation schemes described above, this compensation approach has its own limitations. For example, each mirror can provide only a limited and fixed quanta of dispersion compensation. As a result, the mirror dispersion can only be varied in discrete dispersion quanta, and a complicated system of intracavity mirrors is required to obtain a desired net dispersion.